Traffic signals in use today generally utilize an incandescent bulb positioned behind a green, yellow or red plastic lens. The bulb in each individual traffic signal assembly is illuminated at an appropriate time by an electronic controller, which often operates under the control of a timer. For example, the traffic signal that utilizes the green lens is illuminated for 45 seconds, the "yellow" signal is then illuminated for 5 seconds, and the "red" signal then is illuminated for 45 seconds. Alternatively, the "green" signal can be illuminated continuously until such time as a vehicle approaches the intersection from a cross-street, as sensed by a sensor element in the road, at which time the "green" light is turned off and the "yellow" light is illuminated, and then the "red". Other ways to control the illumination of particular signal lights are well known.
Present technology in use for traffic signals has five main areas where improvements are desirable, namely, (1) improved "readability" in sunlight, (2) lower power requirements, (3) reduction of glare at night, (4) improved redundancy, and (5) reduced weight and wind resistance.
The term "readability" refers in part to the situation where the sunlight is shining directly into the signal and the observer looks directly at the signal. In this situation, it is difficult for the observer to easily "read" or "distinguish" the signal information. In other words, the observer has a difficult time determining which of the signal lights is illuminated. The observer can see the signals but cannot easily differentiate between one that is "on" and ones that are "off".
When the sun is shining directly into the signal, the sunlight is reflected from the front surface of the signal (which is usually colored plastic), directly into the observer's eyes. Because the sunlight is much more intense than the light emitted from the "on" signal, all signal surfaces are reflecting the sunlight and may appear to be "on". The observer may be unable to differentiate between that signal which is actually "on" and those signals which are "off". Again, there are two ways to address this problem, namely either increasing the brightness of the "on" signal or somehow significantly reducing the reflected brightness of the "off" signals.
Observation of traffic signals at night presents a different problem, specifically the problem of glare. Glare consists of too much light or brightness emitted from the "on" signal. This produces excessive reflections from the road, especially when wet, and from the housing structure surrounding the signal. The result is that the observer has more difficulty in reading the signal because of the added distractions in his viewing area.
Obviously, increasing the brightness of the "on" signal, a possible solution to the problem of sunlight readability, is not a sufficient solution to the problem of night time glare. Further, increasing the brightness of the "on" signal may involve increasing the intensity of the incandescent lamp, requiring more electrical energy and reducing the lifetime of the lamp.
"Redundancy" relates to the ability of the traffic signal to remain readable when the incandescent lamp fails. In conventional traffic signals, if the lamp fails, the observer cannot determine which signal is "on", as none of the individual signals in the traffic light are illuminated.
The need for a traffic signal having reduced weight and wind resistance refers to the fact that a heavy, large surface area hood is secured to the front of most conventional traffic signals in order to cut down on glare and to help shade the lens from sunlight, thereby making the signal more readable.
In order to overcome these problems, a preferred embodiment of the lens assembly of the present invention uses dichroic liquid crystal technology to increase the perceived differences between the "on" and "off" signals by increasing the relative contrast between signal lights, rather than just increasing the brightness or intensity of the "on" signal.
Contrast concerns the condition of one signal relative to another and thus describes the performance of the signals. There are two types of contrast important in the context of traffic signals. One is brightness contrast and the other is color contrast. Brightness contrast is simply the ratio between the brightness emitted from one signal relative to another. If one signal emits three times as much light as the other, the contrast between the two signals is 3:1 or just 3.
Color contrast is primarily described by two parameters, brightness and color difference. Both of these parameters are measurable. Two signals of the same color and same measured brightness cannot be distinguished from one another by an observer. If the same signals are now different colors and still have equal brightness, an observer will distinguish a difference between the signals if the color difference is sufficient. The difficulty is that the resulting color contrast measurements do not always correctly describe whether or not the color contrast is good. In the specific application where only three distinct, well defined colors are being used, such as in a green, yellow and red traffic signal, a qualitative description of the color contrast will suffice.
The lens assembly of the present invention enhances both brightness and color contrast of traffic signals.
One attempt to improve the visibility of traffic lights is shown in U.S. Pat. No. 4,791,418 to Kawahara. In the '418 patent, a liquid crystal device is attached to a cylindrical hood on the front of the signal light, in front of a colored plate or cover that is attached to the casing of the signal. The colored plate or cover in turn is positioned in front of the lamp for the signal. The liquid crystal device has an encapsulated liquid crystal material with a dye, such that incident light is scattered when no electric field is applied to the liquid crystal, and transmits incident light when an electric field is applied. Transparent electrodes on either side of the liquid crystal material are used to apply the electric field. Power is supplied to the electrodes at the same time power is applied to the lamp for the signal. A battery is shown as the power source, although an alternating current source is also mentioned in the '418 patent specification.
In order to increase the visual distinguishability of the colored plate that is located in front of the bulb for the signal light, the '418 states that a surface of the colored plate can be coated with a transparent light-scattering paint. The '418 patent also states that a light transmitting body coated with a transparent light-reflecting film may be placed between the lamp and the transparent color plate. Alternatively, the transparent light-reflecting film can be applied to the interior surface of the transparent colored plate.
The '418 patent also mentions applying film on the outer surface of the liquid crystal device for absorbing ultraviolet light, and also applying a non-reflecting light film to the liquid crystal device. The '418 patent also states that the non-reflecting light film may be coated onto the transparent electrodes.
The '418 patent further states that, in place of an encapsulated liquid crystal, a polarizer, analyzer and a liquid crystal exhibiting twisted nematic effects may be used.
Another traffic signal that uses a liquid crystal shutter is shown in U.S. Pat. No. 4,652,851 to Lewin, which discloses a traffic signal that uses one, or a group of, continuously illuminated fluorescent or HID lamp(s) and attenuation devices on each signal. One form of attenuation device identified in the '851 patent is a liquid crystal panel, where the panel is either opaque or clear, depending on whether an electric field has been applied to the liquid crystal panel. Color for the signals is provided by a colored lens on the outside of the signal or by a combination of colored fluorescent lamps and color filters or lenses.
The apparatus of the present invention, which is a lens assembly that can be used to completely replace the colored lens of a conventional traffic signal, provides for enhanced visibility and improved redundancy of the signal, and also overcomes certain other failings in known prior art systems. The method of the present invention represents an improved way of converting a light source into a signal indicator using an electro-optical shutter, for example, a shutter that utilizes dichroic liquid crystal material.